1. Field of the Invention
The present invention relates to a display device in which a transistor is provided for each pixel and a driving method thereof.
2. Description of the Related Art
Since display devices using light-emitting elements have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angles, they have attracted attention as display devices which can take the place of CRTs (cathode ray tubes) or liquid crystal display devices. Specifically proposed structures of active matrix display devices using light-emitting elements are different depending on manufacturers. However, in general, at least a light emitting element, a transistor (a switching transistor) which controls input of video signals to pixels, and a transistor (a driving transistor) which controls value of current supplied to the light-emitting elements are provided for each pixel.
On the other hand, thin film transistors have an advantage that the thin film transistors can be formed over glass substrates which are inexpensive while the thin film transistors have a disadvantage that characteristics such as a threshold voltage and a mobility are likely to vary compared to bulk transistors. Since the value of a current which flows through driving transistors relates to values of a threshold voltage and mobility, variations in mobility and the threshold voltage are reflected to luminance of light-emitting elements in consequence. Therefore, it is an important object to propose a driving method for improving the image quality of display devices in which current value of driving transistors can be corrected in expectation of variations in the threshold voltage and mobility.
In Patent Document 1 (Japanese Published Patent Application No. 2006-084899), a driving method for a display device, in which not only the threshold voltage but also mobility can be corrected, has been proposed. In the driving method disclosed in Patent Document 1, during a detection period, a potential difference between a gate and a source of a drive transistor which corresponds to a driving transistor is held in a capacitor portion before the potential difference becomes equal to the threshold voltage. With the above-described structure, not only a variation in the threshold voltage but also a variation in the mobility can be prevented from influencing luminance.